Systems and methods for transmitting data frames using multiple antennas

ABSTRACT

A transmitter including a transmit section and a frame formatter. The transmit section is to generate a stream of data to be transmitted using a plurality of antennas and separate the stream of data into a plurality of substreams corresponding to respective ones of the plurality of antennas. The frame formatter is to form respective frames of data from the plurality of substreams. Each of the frames of data includes a short preamble including short training symbols, a long preamble including long training symbols, and a signal field indicating a quantity of the plurality of antennas. The transmit section is further to transmit the frames of data using respective ones of the plurality of antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser. No. 13/727,882 (now U.S. Pat. No. 9,191,051), filed Dec. 27, 2012, which is a continuation of Ser. No. 13/206,105 (now U.S. Pat. No. 8,345,534), filed on Aug. 9, 2011, which is a divisional of U.S. patent application Ser. No. 10/896,313, filed on Jul. 20, 2004 (now U.S. Pat. No. 7,995,455), which claims the benefit of U.S. Provisional Application Ser. No. 60/537,973, filed on Jan. 21, 2004, and to U.S. Provisional Application Ser. No. 60/538,026, filed on Jan. 21, 2004. The entire disclosures of the applications referenced above are incorporated herein by reference.

BACKGROUND

Wireless phones, laptops, PDAs, base stations and other systems may wirelessly transmit and receive data. A single-in-single-out (SISO) system may have two single-antenna transceivers in which one predominantly transmits and the other predominantly receives. The transceivers may use multiple data rates depending on channel quality.

An M_(R)×M_(T) multiple-in-multiple-out (MIMO) wireless system uses multiple transmit antennas (M_(T)) and multiple receive antennas (M_(R)) to improve data rates and link quality. The MIMO system may achieve high data rates by using a transmission signaling scheme called “spatial multiplexing,” where a data bit stream is demultiplexed into parallel independent data streams. The independent data streams are sent on different transmit antennas to obtain an increase in data rate according to the number of transmit antennas used. Alternatively, the MIMO system may improve link quality by using a transmission signaling scheme called “transmit diversity,” where the same data stream (i.e., same signal) is sent on multiple transmit antennas after appropriate coding. The receiver receives multiple copies of the coded signal and processes the copies to obtain an estimate of the received data.

The number of independent data streams transmitted is referred to as the “multiplexing order” or spatial multiplexing rate (r_(s)). A spatial multiplexing rate of r_(s)=1 indicates pure diversity and a spatial multiplexing rate of r_(s)=min(M_(R), M_(T)) (minimum number of receive or transmit antennas) indicates pure multiplexing.

SUMMARY

A multiple-in-multiple-out (MIMO)/orthogonal frequency-division multiplexing (OFDM) system may use different types of space-frequency code matrices for encoding data on multiple substreams for transmission on multiple antennas. The system may utilize a MIMO-OFDM frame format that includes additional long training OFDM symbols for training additional antennas and for link adaptation and a header with an additional SIGNAL symbol to indicate MIMO-OFDM-specific information.

The system may include a code module to generate a codeword vector for a tone in an OFDM symbol from a vector corresponding to the tone and a space-frequency code matrix corresponding to a particular spatial multiplexing rate. The space-frequency code matrix may enable the system to transmit tones in the OFDM symbol with equal transmit power per tone from the antennas, e.g., by using a matrix such that the Frobenius norm of the space-frequency code matrix is constant for all tones. The space-frequency code matrix may enable the system to transmit tones in the OFDM symbol with equal power per antennas, e.g., by using a matrix in which the row norm is equal for each row. The code module may employ a permuted space-frequency coding technique in which the code module cycles through permutations on a tone-by-tone basis. Alternatively, the code module may employ a generalized cyclic delay diversity technique in which the space frequency code matrix is given by the following equation: B_(k)=k_(M) _(T) _(,r) _(s) D_(k)F_(M) _(T) _(,r) _(s) , where k_(M) _(T) _(,r) _(s) is the normalization constant, F_(M) _(T) _(,r) _(s) is a Fourier sub-matrix consisting of the first r_(s) columns of an M_(T)-point discrete Fourier transform, and D_(k) is a diagonal matrix of exponentials that are a function of a cycle delay on each of the plurality of antennas, and is given by the equation: D_(k)=diag{e^(−j2πkL) ^(i) ^(/N)}_(i=0) ^(M) ^(T) ⁻¹, where L_(i) is a cyclic delay for the i-th antenna, and N is the size of an inverse fast Fourier transform (IFFT).

The system may utilize an IEEE 802.11a-compliant frame format or a MIMO-OFDM frame format. The MIMO-OFDM frame format may include a short preamble in which tones are distributed across the antennas in a cyclic manner. The MIMO-OFDM frame format may include multiple long training symbols, e.g., one for each antenna, and an additional SIGNAL symbol including MIMO-OFDM specific information, e.g., the spatial multiplexing rate and the number of transmit antennas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a wireless MIMO-OFDM communication system according to an embodiment.

FIG. 2 is a block diagram of a transmit section in a transceiver in the MIMO-OFDM communication system.

FIG. 3 is a block diagram of the receive section in a transceiver in the MIMO-OFDM communication system.

FIG. 4 is block diagram of a portion of a space-frequency code module in the transmit section of FIG. 2.

FIG. 5 illustrates an IEEE 802.11a frame format.

FIG. 6 illustrates a frame format for the MIMO-OFDM communication system.

FIGS. 7A-7D show examples of a transmitted MIMO-OFDM short training symbol in the frequency domain for MIMO-OFDM systems utilizing one, two, three, and four transmit antennas, respectively.

FIG. 8 is a flowchart describing a space-frequency coding operation according to an embodiment.

FIG. 9 is a flowchart describing the operation of the frame formatter in an IEEE 802.11a-compliant mode and a MIMO-OFDM mode.

DESCRIPTION

FIG. 1 illustrates a wireless multiple-in-multiple-out (MIMO) communication system 100, which includes a first transceiver 102 with M_(T) transmit (T_(X)) antennas 104 and a second transceiver 106 with M_(R) receive (R_(X)) antennas 108, forming an M_(R)×M_(T) MIMO system. For the description below, the first transceiver 102 is designated as a “transmitter” because the transceiver 102 predominantly transmits signals to the transceiver 106, which predominantly receives signals and is designated as a “receiver”. Despite the designations, both “transmitter” 102 and “receiver” 106 may include a transmit section 110 and a receive section 112 and may transmit and receive data.

The transmitter 100 and receiver 102 may be implemented in a wireless local Area Network (WLAN) that complies with the IEEE 802.11 standards (including IEEE 802.11, 802.11a, 802.11b, 802.11g, and 802.11n). The IEEE 802.11 standards describe orthogonal frequency-division multiplexing (OFDM) systems and the protocols used by such systems. In an OFDM system, a data stream is split into multiple substreams, each of which is sent over a different subcarrier frequency (also referred to as a “tone”). For example, in IEEE 802.11a systems, OFDM symbols include 64 tones (with 48 active data tones) indexed as {−32, −31 , . . . , −1,0,1, . . . , 30,31}, where 0 is the DC tone index. The DC tone is not used to transmit information.

The antennas in the transmitter 102 and receiver 106 communicate over channels in a wireless medium. In FIG. 1, H represents the reflections and multi-paths in the wireless medium, which may affect the quality of the channels. The system may perform channel estimation using known training sequences which are transmitted periodically (e.g., at the start of each frame). A training sequence may include one or more pilot symbols, i.e., OFDM symbols including only pilot information (which is known a priori at the receiver) on the tones. The pilot symbol(s) are inserted in front of each transmitted frame. The receiver 106 uses the known values to estimate the medium characteristics on each of the frequency tones used for data transmission. For example, on the receiver side, the signal Y_(k) for tone k in an SISO system can be written as, Y _(k) =H _(k) Q _(k) +N _(k),

where H_(k) is the channel gain for the k^(th) tone, Q_(k) is the symbol transmitted on the k^(th) tone, and N_(k) is the additive noise. An estimate of the channel may be determined at the receiver by dividing Y_(k) by Q_(k).

The number of independent data streams transmitted by the transmit antennas 104 is called the “multiplexing order” or “spatial multiplexing rate” (r_(s)). A spatial multiplexing rate of r_(s)=1 indicates pure diversity, and a spatial multiplexing rate of r_(s)=min(M_(R), M_(T))(minimum number of receive or transmit antennas) indicates pure multiplexing.

In an embodiment, the MIMO system 100 may use combinations of diversity and spatial multiplexing, e.g., 1≤r_(s)≤min(M_(R), M_(T)). For example, in a 4×4 MIMO system, the system may select one of four available multiplexing rates (r_(s) ϵ [1,2,3,4]) depending on the channel conditions. The system may change the spatial multiplexing rate as channel conditions change.

FIG. 2 shows a block diagram of the transmit section 110. The transmit section 110 includes stages similar to those in the transmit section of an IEEE 802.11a transmitter, but with some modifications to account for the multiple transmit antennas.

Data bits in a data stream to be transmitted are scrambled by a scrambler 200 and then encoded by a convolutional encoder 202. A space-frequency interleaver 204 interleaves and separates the single data stream into M_(T) substreams. Quadrature amplitude modulation (QAM) constellation mapping may be performed on each substream by mapping modules 206. The mapped substreams are then encoded using a linear space-frequency coding module 208, the operation of which is described in more detail below. Each of the individual substreams are processed in a corresponding processing chain 210. Each processing chain includes a pilot symbol insertion module 212, a serial-to-parallel (S/P) converter 214, an inverse fast Fourier transform (IFFT) module 216, a parallel-to-serial (P/S) converter 218, a cyclic prefix module 220, and a radio frequency (RF) module 222 for digital-to-analog (D/A) and baseband-to-RF conversion.

FIG. 3 shows a block diagram of the receive section 112. The receive section 112 includes stages similar to those in the receive section of an IEEE 802.11a receiver, but with some modifications to account for the multiple receive antennas.

Signals received on the multiple receive antennas are input to corresponding processing chains 300. Each processing chain includes an RF module 301 for RF-to-baseband and analog-to-digital (A/D) conversion. A time/frequency synchronization module 302 performs synchronization operations and extracts information from the multiple substreams for channel estimation 303. Each processing chain 300 includes a cyclic prefix removal module 304, S/P converter 306, fast Fourier transform (FFT) module 308, a common phase error (CPE) correction module 310, a space-frequency detection module 312, and a P/S converter 314. The multiple substreams are input to a space-frequency deinterleaver and decoding module 316 which de-interleaves the substreams into a single data stream 317 and performs soft decoding. The single stream is then input to a descrambler 318.

The space-frequency code module 208 in the transmit section 110 generates codewords for the different substreams. For a given spatial multiplexing rate (r_(s)), a vector X of modulated symbols for each tone is defined, as shown in FIG. 4. For the k-th tone, this vector is given by: X_(k)=[X_(0,k) . . . X_(r) _(s) _(−1,k)]^(T).

The codeword vector C_(k) for the k-th tone is given by: C_(k)=[C_(0,k) . . . C_(M) _(T) _(−1,k)]^(T).

The codeword vector C_(k) may be calculated in a codeword generator 402 by multiplying the vector X_(k) by the M_(T)×r_(s) space-frequency code matrix for the k-th tone (B_(k)), i.e., C_(k)=B_(k)X_(k).

The codeword C_(k) is output to the processing chains 210, and an IFFT operation is performed on the codeword vector to generate the time domain codeword c_(n), where c_(n)=[c_(0,n) . . . c_(M) _(T) _(−1,n)]^(T).

In an embodiment, the code matrix B_(k) is selected such that there is equal transmit power per tone and equal transmit power for each antenna. To produce equal transmit power per tone, the sum of the squares of all elements in the matrix, i.e., the Frobenius norm of code matrix B_(k), must constant for all tones, i.e., ∥B_(k)∥_(F) ²=const.,k=0, . . . , N_(u)−1

where N_(u) is the number of used tones (subcarriers), e.g., 52 tones for IEEE 802.11a compatible systems.

To produce equal transmit power per antenna, the row norms summed across all tones must be equal for each of the rows, i.e.,

${\sum\limits_{k = 0}^{N_{u} - 1}{\left( {B_{k}B_{k}^{H}} \right)i}},{i = {{const}.}},{i = 0},\ldots\mspace{14mu},{M_{T} - 1.}$

Utilizing equal transmit power per antenna may be useful in minimizing the cost of the power amplifier (PA) in the transmitter.

Different types of space-frequency encoding may be implemented by the space-frequency code module 208. In an embodiment, permuted space-frequency codes (also referred to as “antenna tone hopping”) may be used. In antenna tone hopping, symbols are transmitted on the multiple transmit antennas such that each tone has only one antenna active for r_(s)=1. The space-frequency code module 208 may cycle through different permutations of tone-antenna mappings, e.g., on a tone-by-tone basis. For example, for a system with three transmit antennas (M_(T)=3) and using pure diversity spatial multiplexing (r_(s)=1), the code matrix is given by

${B_{k} \in \left\{ {{c_{31}\begin{bmatrix} 1 \\ 0 \\ 0 \end{bmatrix}},{c_{31}\begin{bmatrix} 0 \\ 1 \\ 0 \end{bmatrix}},{c_{31}\begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix}}} \right\}},$

where c₃₁ is a normalization constant. In this example, the permutations include, from left to right, (1) only antenna T×0 transmitting tone k, (2) only antenna T×1 transmitting tone k, and (3) only antenna T×2 transmitting tone k, respectively. The space-frequency code module 208 may cycle through these permutations on a tone-by-tone basis.

Similarly, for a system with M_(T)=3 and r_(s)=2,the code matrix is given by

${B_{k} \in \left\{ {{c_{32}\begin{bmatrix} 1 & 0 \\ 0 & 1 \\ 0 & 0 \end{bmatrix}},{c_{32}\begin{bmatrix} 0 & 0 \\ 1 & 0 \\ 0 & 1 \end{bmatrix}},{c_{32}\begin{bmatrix} 0 & 1 \\ 0 & 0 \\ 1 & 0 \end{bmatrix}}} \right\}},$

wherein c₃₂ is a normalization constant. In this example, the permutations include, from left to right, (1) antenna T×0 transmitting tone k in the first substream and antenna T×1transmitting tone k in the second substream, (2) antenna T×1 transmitting tone k in the first substream and antenna T×2 transmitting tone k in the second substream, and (3) antenna T×2 transmitting tone k in the first substream and antenna T×0 transmitting tone k in the second substream, respectively. Other permutations may be possible.

Another type of space-frequency encoding that may be implemented by the space-frequency code module 208 is generalized cyclic delay diversity. In this encoding technique, substreams are switched between antennas on a cyclic basis. For example, symbol A may be transmitted on antenna T×0, and then on antenna T×1 after a cyclic delay L₂, and then on antenna T×2 after a cyclic delay L₃. The code matrix B_(k) is given by: B_(k)=k_(M) _(T) _(,r) _(s) D_(k)F_(M) _(T) _(,r) _(s)

where k_(M) _(T) _(,r) _(s) is the normalization constant, D_(k) is a diagonal matrix, and F_(M) _(T) _(,r) _(s) is a Fourier sub-matrix consisting of the first r_(s) columns of an M_(T)-point discrete Fourier transform matrix. In addition to the Fourier matrix, other matrices may be used, including Vandermonde, Hadamard, or Walsh matrices. D_(k) is a diagonal matrix of exponentials that are a function of the cycle delay on each of the transmit antennas, and is given by D_(k)=diag{e^(−j2πkL) ^(i) ^(/N)}_(i=0) ^(M) ^(T) ⁻¹,

where L_(i) is the cyclic delay for the i-th antenna, and N is the size of the IFFT.

The MIMO-OFDM system may be compatible with IEEE 802.11a systems, and consequently may have many similarities to an IEEE 802.11a system. For example, like IEEE 802.11a systems, the MIMO-OFDM system may use 52 tones (48 data tones and 4 pilot tones), 312.5 kHz subcarrier spacing, a FFT/IFFT period of 3.2 μs, a cyclic prefix with a duration of 0.8 μs, and an OFDM symbol duration of 4.0 μs. The MIMO-OFDM system may also use a frame format similar to that specified by IEEE 802.11a, which is shown in FIG. 5. In addition, variations of the MIMO-OFDM systems are also possible, including using different numbers of tones, different guard intervals, different forward error correction codes, and different constellations.

An IEEE 802.11a frame 500 includes a short preamble 501, a long preamble 502, a header 504, and a DATA field 506. The short preamble 501 consists of a short training symbol 508 with a duration of 0.8 μs repeated ten times. The short preamble 501 is used for signal detection, automatic gain control (AGC), coarse frequency offset estimation, and symbol timing estimation.

The long preamble 502 includes two long training symbols 510, each of duration 3.2 μs, which are separated from the short training symbols 508 by a long guard interval (1.6 μs) 512.

The header 504 includes a SIGNAL symbol 514, which is encoded at 6 Mbps. The SIGNAL symbol 514 is 12 bits in length and includes 4 bits for the data rate, 1 reserved bit, 1 parity bit, and 6 tail bits (set to “0” to return the convolutional decoder to State 0).

The DATA field 506 includes OFDM symbols including the data bits to be transmitted. The data bits are prepended by a 16-bit SERVICE field and are appended by 6 tail bits. The resulting bits are appended by a number of pad bits needed to yield an integer number of OFDM symbols.

The MIMO-OFDM system may use a similar frame format, as shown in FIG. 6. The illustrated frame format 600 is for systems with three transmit antennas (M_(T)=3), but can be modified for other M_(T). A frame formatter 120 (FIG. 1) in the transmitter 102 may use components in the transmit data path shown in FIG. 2 to generate and process different sections of the frame for transmission from the transmit antennas.

Each transmit antenna transmits a different MIMO-OFDM frame 600. Like the IEEE 802.11a frame 500, the MIMO-OFDM frames 600 include a short preamble 602 with a series of short training symbols 604, a long preamble 605 with a set of two long training symbols 606, a header 608 including a SIGNAL symbol 610, and a data field 612. In addition, the header 608 may include a second SIGNAL symbol (SIGNAL2) 614, which may be used to transmit MIMO-OFDM-specific information, such as the number of transmit antennas and the spatial multiplexing rate. The frame may also include a supplemental long preamble 616 including M_(T) −1 additional long training symbols to train the other antennas.

As in IEEE 802.11a, a short OFDM training symbol consists of 12 tones, which are modulated by the elements of the following frequency-domain sequence:

$S_{{- 26},26} = {\sqrt{\frac{13}{6}} \times \left\{ {0,0,{1 + j},0,0,0,{{- 1} - j},0,0,0,{1 + j},0,0,0,{{- 1} - j},0,0,0,{{- 1} - j},0,0,0,{1 + j},0,0,0,0,0,0,0,{{- 1} - j},0,0,0,{{- 1} - j},0,0,0,{1 + j},0,0,0,{1 + j},0,0,0,{1 + j},0,0,0,{1 + j},0,0} \right\}}$

The multiplication by √{square root over (13/6)} is in order to normalize the average power of the resulting OFDM symbol. The short training symbol has a duration of 0.8 μs and is repeated 10 times.

In an embodiment, different tones in the short training symbol may be transmitted on different antennas. FIGS. 7A-7D show examples of a transmitted MIMO-OFDM short training symbol in the frequency domain for M_(T)=1,2,3,4, respectively.

For M_(T)=1,all tones are transmitted on one antenna, T×0, as shown in FIG. 7A. The short training symbol in this case is the same as the IEEE 802.11a short training symbol. For M_(T)>1, a cyclic transmission technique may be used in which the tones are transmitted on different antennas in a cyclic manner. As shown in FIG. 7B, for M_(T)=2,six subcarriers are transmitted on each of the two antennas, T×0 and T×1. Similarly, for M_(T)=3, four subcarriers are transmitted on each of antennas T×0, T×1, and T×2, as shown in FIG. 7C, and for M_(T)=4, three subcarriers are transmitted on each of the antennas T×0, T×1, T×2, and T×3. In this manner, the short training symbols may be transmitted with equal power per transmit antenna, which may facilitate the use of low-power power amplifiers in the transmitter. Also, the cyclic transmission technique produces no cross-correlation across transmit antennas, which may facilitate improved AGC performance at the receiver. The short preamble 602 is compatible with IEEE 802.11a receivers for M_(T)≥1 because the standard 802.11a receiver can receive and use all tones regardless of whether they were transmitted on one or more antennas.

As in IEEE 802.11a, the long preamble 602 is used for fine frequency offset estimation and channel estimation. A long training OFDM symbol includes 52 tones, which are modulated by the following frequency-domain BPSK training sequence: L_(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1−1,−1,1,1,−1,1,−1,1,1,1,1,0,1, −1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1 −1,1,−1,1,1,1,1}

The number of sets of long training symbols may be M_(T) for all spatial multiplexing rates. The additional long training symbols may be used to estimate the full M_(R)×M_(T) channel matrix. This estimation may be used for link adaptation, in which modulation, coding rate, and/or other signal transmission parameters may be dynamically adapted to the changing channel conditions. However, for compatibility with IEEE 802.11a systems, the transmitter may also operate in a legacy mode in which only one set of long training symbols is transmitted on all antennas, i.e., the supplemental long preamble 616 is omitted. In the legacy mode, the SIGNAL2 symbol 614 may also be omitted, such that the data starts after the SIGNAL symbol 610.

The preamble may be generated from r_(s) frequency domain input symbols, p=0, . . . , r_(s−1). For data tones in the long preamble section, the vector X is given by: X_(k) ^((p))=[0 . . . 0 L_(k) 0 . . . 0]^(T),

where L_(k) is positioned at the p-th position in the vector.

For pilot tones in the long preamble section, the vector X is given by: X_(k) ^((p))=[L_(k) 0 . . . 0]^(T)

The frequency-domain preamble may then be generated by multiplying the vector X by the matrix B_(k), i.e., C_(k) ^((p))=B_(k) ^((p))X_(k) ^((p))

The time-domain preamble p for the i-th antenna is given by: c_(p,n) ^((i)).

The additional long training symbols in the supplemental long preamble 616 may be generated from input symbols p=r_(s), . . . , M_(T)−1 by multiplying L_(k) with (M_(T)−r_(s))M×1 mutually orthogonal column vectors that are orthogonal to the columns of B_(k).

Data bits in the data field 612 are scrambled, convolutionally encoded, and interleaved before mapping onto subcarrier constellations in a manner similar to that specified in IEEE 802.11a. To account for the availability of multiple spatial multiplexing rates, the interleaving may be performed by a block interleaver with a block size equal to the number of coded bits per spatial substream in an OFDM symbol.

As in IEEE 802.11a, tones k=−21, −7, 7, and 21 are used for pilot tones in each data MIMO-OFDM symbol. The pilot tones consist of modulating the first spatial substream with IEEE 802.11a MPSK pilot tone sequence followed by multiplication by code matrix B_(k). The dimension of B_(k) for pilot tones is M_(T)×1. The matrix B_(k) for each pilot tones may be selected to maintain equal power per transmit antenna.

FIG. 8 is a flowchart describing a space-frequency coding operation 800 according to an embodiment. The codeword generator 402 (FIG. 4) in the space-frequency code module 208 receives a vector for the n-th tone having r_(s) modulated symbols (block 802). The codeword generator 402 multiplies the vector for the k-th tone by the space-frequency code matrix B_(k) (block 804). As described above, B_(k) is selected to produce equal transmit power for each tone and equal transmit power for each antenna, and may be different depending on the type of space-frequency encoding being performed (e.g., antenna-tone hopping or generalized cyclic delay diversity). The codeword generator 402 then outputs the frequency domain codeword vector for the k-th tone (block 806), which has M_(T) symbols corresponding to the M_(T) transmit antennas.

As described above, the frame formatter 120 MIMO-OFDM transmitter may be operated in a MIMO-OFDM mode and a legacy mode for compatibility with IEEE 802.11a systems. FIG. 9 is a flowchart describing the operation of the frame formatter in the two modes. If the frame formatter is in the legacy mode (block 902), the frame formatter generates an IEEE 802.11a-compliant frame (block 904). The frame has a format in which different tones in the short training symbol may be transmitted on different antennas, e.g., in a cyclic distribution (block 906). In the IEEE 802.11a-compliant mode, the transmitter may transmit only one set of long training symbols (block 908) and a header with only one SIGNAL symbol (block 910). Finally, the transmitter transmits the frame's DATA field (block 912).

If the frame formatter is in the MIMO-OFDM mode (block 902), the frame formatter generates a MIMO-OFDM frame (block 914). The frame has a format in which different tones in the short training symbol may be transmitted on different antennas, e.g., in a cyclic distribution (block 916). In the MIMO-OFDM mode, the transmitter may transmit multiple sets of long training symbols, one for each of the M_(T) transmit antennas (block 918) to facilitate link adaptation and a header with only two SIGNAL symbols (block 920), including one SIGNAL symbol with MIMO-OFDM-specific information. Finally, the transmitter transmits the frame's DATA field (block 922).

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, blocks in the flowcharts may be skipped or performed out of order and still produce desirable results. Accordingly, other embodiments are within the scope of the following claims.

Embodiments of the subject matter described in this specification can be implemented as one or more computer programs embodied in a machine-readable medium to control the operation of one or more machines, including a computer, to cause the one or more machines to perform operations as described in this specification. As used herein, a machine-readable medium does not include a propagated signal and is a non-transitory medium. 

What is claimed is:
 1. A transmitter, comprising: a transmit section to generate a stream of data to be transmitted using a plurality of antennas, and separate the stream of data into a plurality of substreams corresponding to respective ones of the plurality of antennas; and a frame formatter to form respective frames of data from the plurality of substreams, wherein each of the frames of data includes a short preamble including short training symbols, a long preamble including long training symbols, and a signal field indicating a quantity of the plurality of antennas and a spatial multiplexing rate, the frame formatter further to vary the spatial multiplexing rate as indicated n the signal field between 1 and the quantity of the plurality of antennas, wherein the transmit section is further to transmit the frames of data using respective ones of the plurality of antennas in accordance with the spatial multiplexing rate as indicated in the signal field, and wherein, to transmit the frames of data, the transmit section is further to generate a space-frequency code matrix such that (i) the substreams are transmitted at an approximately equal power per tone and (ii) the substreams are transmitted at an approximately equal power per transmit antenna.
 2. The transmitter of claim 1, wherein the long training symbols include (i) a first set of long training symbols associated with training a first antenna of the plurality of antennas and (ii) a second set of long training symbols associated with training at least one second antenna of the plurality of antennas.
 3. The transmitter of claim 1, wherein, to transmit the frames of data, the transmit section is further to transmit different tones in the short training symbols using the respective ones of the plurality of antennas.
 4. The transmitter of claim 3, wherein, to transmit the different tones, the transmit section is further to transmit the different tones in a cyclic manner using the respective ones of the plurality of antennas.
 5. The transmitter of claim 1, wherein, to transmit the frames of data, the transmit section is further to transmit the short training symbols using the respective ones of the plurality of antennas.
 6. The transmitter of claim 1, wherein a quantity of sets of the long training symbols corresponds to the quantity of the plurality of antennas.
 7. The transmitter of claim 1, the transmit section is further to generate the space-frequency code matrix such that a Frobenius norm of the space-frequency code matrix is constant for all tones.
 8. A method of operating a transmitter, the method comprising: generating a stream of data to be transmitted using a plurality of antennas; separating the stream of data into a plurality of substreams corresponding to respective ones of the plurality of antennas; forming respective frames of data from the plurality of substreams, wherein each of the frames of data includes a short preamble including short training symbols, a long preamble including long training symbols, and a signal field indicating a quantity of the plurality of antennas and a spatial multiplexing rate, wherein forming the respective frames includes variably controlling the spatial multiplexing rate as indicated in the signal field between 1 and the quantity of the plurality of antennas; and transmitting the frames of data using respective ones of the plurality of antennas in accordance with the spatial multiplexing rate as indicated in the signal field, wherein transmitting the frames of data includes generating a space-frequency code matrix such that (i) the substreams are transmitted at an approximately equal power per tone and (ii) the substreams are transmitted at an approximately equal power per transmit antenna.
 9. The method of claim 8, wherein the long training symbols include (i) a first set of long training symbols associated with training a first antenna of the plurality of antennas and (ii) a second set of long training symbols associated with training at least one second antenna of the plurality of antennas.
 10. The method of claim 8, wherein transmitting the frames of data includes transmitting different tones in the short training symbols using the respective ones of the plurality of antennas.
 11. The method of claim 10, wherein transmitting the different tones includes transmitting the different tones in a cyclic manner using the respective ones of the plurality of antennas.
 12. The method of claim 8, wherein transmitting the frames of data includes transmitting the short training symbols using the respective ones of the plurality of antennas.
 13. The method of claim 8, wherein a quantity of sets of the long training symbols corresponds to the quantity of the plurality of antennas.
 14. The method of claim 8, the space-frequency code matrix is generated such that a Frobenius norm of the space-frequency code matrix is constant for all tones. 